The World Health Organization declared Monday that explosive growth of the mosquito-borne Zika virus — which has been spreading rapidly in the Americas and may be linked to birth defects — constitutes an international public health emergency, signaling a new phase in the global effort to battle the virus.

The United Nations health agency made the decision after convening a panel of experts in Geneva amid reports from Brazil linking the virus to microcephaly, a birth defect in which babies are born with abnormally small heads and brains.

The recent cluster of microcephaly cases and other neurological disorders reported in Brazil followed a similar cluster in French Polynesia in 2014, WHO Director-General Margaret Chan said in a statement.

"A coordinated international response is needed to improve surveillance, the detection of infections, congenital malformations and neurological complications; to intensify the control of mosquito populations; and to expedite the development of diagnostic tests and vaccines to protect people at risk, especially during pregnancy," Chan said.

The illness from the Zika virus is not considered serious, and symptoms — including rashes, joint pain and reddened eyes — are usually mild and last for several days or a week, experts say. The WHO has said that it is aware of no deaths attributed to the virus.

Still, the reports from Brazil of links to birth defects have triggered alarm. There is no vaccine for the virus.

Last week, the WHO declared a threat of "alarming proportions," warning that the Zika virus was "spreading explosively" across the Americas and could infect as many as 4 million people.

Monday's declaration will serve as a kind of global notice of the threat, but the WHO did not move to restrict travel or trade in regions where the virus is found. It will probably trigger additional resources being put toward researching the virus and preventing its spread.

At least 31 cases of Zika have been detected in 11 U.S. states and the District of Columbia since last year, according to Dr. Anne Schuchat, principal deputy director of the Centers for Disease Control and Prevention. All of those people contracted the virus while traveling abroad, including a girl from Los Angeles County who had traveled to El Salvador. So far, there is no evidence of Zika being contracted in the United States.

The CDC is also aware of 19 confirmed cases of Zika in Puerto Rico and one in the U.S. Virgin Islands, Schuchat said last week. It is unclear whether all of those patients contracted Zika while traveling or at home, she said.

The WHO, however, says transmission will probably spread to all the countries and territories in the Americas hosting the Aedes mosquito that transmits the virus, including the U.S. mainland.

Some reports have also linked the Zika virus to cases of Guillain-Barre syndrome, which can cause temporary paralysis.

Among other recommendations, the committee of experts convened by the world health body suggested that surveillance for microcephaly and Guillain-Barre syndrome "should be standardized and enhanced, particularly in areas of known Zika virus transmission and areas at risk of such transmission." The panel also urged additional research into "clusters" of microcephaly and other disorders linked to Zika.

"As these clusters have occurred in areas newly infected with Zika virus, and in keeping with good public health practice and the absence of another explanation for these clusters, the committee highlights the importance of aggressive measures to reduce infection with Zika virus, particularly among pregnant women and women of childbearing age," the panel said.

The Zika illness is common in parts of equatorial Africa and Southeast Asia. Outbreaks have also been reported in the Pacific islands.

But the disease did not begin to spread widely in the Americas until May, when an outbreak was reported in Brazil. It has since spread to 23 countries and territories in the region.

In Brazil, where Zika has taken on the character of a national emergency, health authorities said they plan to deploy 220,000 members of the military on a single day in February to distribute pamphlets across the country to educate people about the risks posed by mosquitoes.

The government of El Salvador, meanwhile, has advised people to put off having children for two years because of the threat.

Residents of Brazil and other affected areas have also been urged to clean up stagnant pools of water and containers in which the mosquitoes tend to breed.

Containers that can hold even small amounts of water — buckets, flowerpots, tires — should be emptied, health authorities say.

Experts say knowledge of the link between the Zika virus and birth defects is evolving and not yet confirmed. But the reported links from Brazil were sufficient to declare an emergency, officials said.

Authorities are urging pregnant women to take several precautions, including delaying travel to areas where the virus is present. Pregnant women living in areas where the virus exists have also been advised to consider a number of protective measures, such as wearing long sleeves and pants and wearing mosquito repellent.

Monday's declaration that the Zika virus was a public health emergency of international concern was the first such determination by the WHO since the 2013 outbreak of Ebola in West Africa.

In the case of Ebola, the U.N. agency was harshly criticized for what detractors called its slow response. Ebola had already killed more than 1,000 people by the time the agency sounded the alarm in August 2014. Ebola has now sickened more than 26,000 people and killed at least 11,316.

Brazilian President Dilma Rousseff has issued a decree allowing public officials to enter abandoned or empty homes by force if necessary as part of the efforts to eradicate the breeding grounds of the Aedes aegypti mosquito, which has been identified as a vector of the Zika virus, as well as of dengue fever, chikungunya and yellow fever.

The new powers, which came into force Monday, are part of a "provisional measure" that allows public officials to request police assistance in carrying out forced entry if necessary, and also authorizes the carrying out of educational campaigns and the creation of public guidelines.

Brazil's efforts to target the mosquito's breeding grounds are mainly focused in the eastern states of Bahia, Pernambuco and Paraiba, whose governors attended a teleconference with the president Friday, also accompanied by the governors of Rio de Janeiro and Sao Paulo states.

At a news conference after the meeting, Rousseff acknowledged that Brazil was "losing the fight against Aedes," but vowed that it would not lose the war. Speaking alongside Health Minister Marcelo Castro, Rousseff said there could be "no contingencies or limits" to the resources the government would make available in the fight against Zika, which she identified as a threat to public health.

Special correspondent Claire Rigby in Sao Paulo, Brazil, and Times staff writers Melissa Healy and Karen Kaplan in Los Angeles contributed to this report.